Because the ultimate fully private and fully secured mobile device is an acid-covered rock. These "privacy features" destroy interoperability and take away user's control, rendering the device even dumber and even less useful than it was before. It could have been done better, in a way that provided security while preserving utility, but - as is the trend - it wasn't.

Privacy? What??!

Each time your app opens and reads a file, the application on other side of ContentProvider can monitor user activity down to number of bytes read. Each time you open a directory, that action can be noted. If anything, that sounds like a privacy nightmare, and will undoubtedly be exploited.

If security gets in the way of usability, everybody loses. It's always a tradeoff, not binary.